1. Field of the Invention
The present invention relates to transceiver circuits for attaching data terminal equipment to a transmission medium; and more particularly to transceivers for attaching a plurality of data terminal equipment at a single location to the transmission cable of the local area network.
2. Description of Related Art
Local area networks are systems for attaching a plurality of data terminal equipment to a transmission medium so that the terminal equipment can communicate amongst each other. Several standard local area networks have evolved, of which a very popular standard is known as Ethernet, or IEEE standard 802.3. Standard 802.3 is a 1persistent carrier sense multiple access media with collision detection (CSMA/CD). In these systems, when a station wants to transmit, it senses the cable. If the cable is busy, the station waits until the cable goes idle, otherwise it transmits immediately. If two or more stations simultaneously begin transmitting on an idle cable, then a collision occurs. All colliding stations then terminate their transmission, wait a random time and repeat the whole process over again.
Standard 802.3 involves typically a 10 megabit per second base band system using 50 ohm coaxial cable. In order to attach to the cable, a transceiver, or medium attachment unit, is used. The transceiver is clamped onto the cable so that its tap makes contact with the inner core of the cable. The transceiver contains electronics that handle carrier detection and collision detection. When a collision is detected, the transceiver puts a special collision signal on the cable to insure that all other transceivers are notified of the collision.
In order to reduce the number of transceivers needed along a cable, some transceivers allow a plurality of data terminal equipment to attach to them. A typical multi-port transceiver circuit in the prior art is illustrated in FIG. 1. As can be seen, the transceiver includes a first port 10 and a second port 11 which are attached by transceiver cables to a data terminal equipment. These ports are also known as attachment unit interfaces (AUI). The first port is coupled through an isolation transformer 12 to electronic signal coupling and collision detector circuit 13. Similarly, the second port 11 is connected through isolation transformer 14 to the electronic signal coupling and collision detector circuit 13. The electronic signal coupling and collision detector circuit 13 is connected to a third isolation transformer 15 which is attached to a cable transceiver interface (CTI) circuit 16.
Interface circuit 16 is typically an integrated circuit, such as the National Semiconductor DP8392A or an equivalent. This transceiver interface chip is adapted to be connected between the coaxial cable and the port for the data terminal equipment on a single port transceiver unit. For a multiple port unit, a separate signal coupling and collision detector circuit 13 is required to detect collisions between the signals transmitted from the first port 10 or the second port 11.
Power is also typically supplied from the data terminal equipment through the ports 10 and 11. However, a power source is schematically illustrated at point 18 in the circuit in FIG. 1. This point is supplied through a DC/DC converter circuit 19 to power the transceiver interface circuit 16.
As can be seen, in order to connect a plurality of ports at a single transceiver in this system, three separate signal isolation transformer interfaces are required, in addition to the power isolation transformer. Each of these signal isolation transformer interfaces requires a path for transmitted signals, received signals and collision signals. Therefore, the multi-port medium attachment unit is quite expensive.
It is desirable to reduce the number of transformers required for coupling a plurality of data terminal equipment through a single transceiver interface, and otherwise simplify the circuit required for this function.